An oil separating chamber for separating oil contained in blow-by gas is arranged outside a transmission member passage connecting a crank shaft with a cam shaft of a valve system and at an abutting surface between a cylinder block and a cylinder head. Partition walls perpendicular to the abutting surface for dividing the oil separating chamber into a plurality of small subchambers are alternatively arranged at the cylinder block and the cylinder head to form a labyrinth structure. The small subchamber at one end of the plurality of small subchambers is provided with a blow-by gas intake passage communicated with the transmission member passage. The small subchamber at the other end is provided with a blow-by gas exhaust passage for returning blow-by gas to an air cleaner. Each of the small subchambers not provided with the blow-by gas intake passage is provided with an oil return passage communicating with the transmission member passage. This arrangement and construction of an oil separator structure for separating oil contained in blow-by gas of an internal combustion engine reduces the number of component parts and the number of assembling steps.